fix(BA-6915): drop the column default before retyping sessions.result

[jopemachine]

📚 Stacked PRs

• fix(BA-6914): stop dropping FKs by a name the database may not use #12910 — BA-6914 fix: migrations that drop a foreign key by a name the DB may not use
• fix(BA-6913): reconcile indexes that only exist on migrated databases #12907 — BA-6913 fix: indexes that only exist on migrated databases
• fix(BA-6915): drop the column default before retyping sessions.result #12912 — BA-6915 fix: drop the column default before retyping sessions.result ← you are here

Merge in order, top-down. Each PR is based on the one above it, so its diff shows only its own layer. The order is fixed by the alembic chain: #12907's migration takes #12910's parent as its down_revision.

Problem

0b1efbb2db84's downgrade recreates the plural sessionresults type and then:

ALTER TABLE sessions ALTER COLUMN result TYPE sessionresults USING result::text::sessionresults

sessions.result carries a default. PostgreSQL cannot cast an existing default across two unrelated enum types, so this aborts before touching a row:

DatatypeMismatchError: default for column "result" cannot be cast automatically to type sessionresults

The migration guards its DDL with a try/except over a list of expected-divergence SQLSTATEs — but 42804 isn't on it, so the error is reraised and the whole downgrade chain stops here.

The correct order already exists in this repo

dec0deba5893 does the same conversion and succeeds, because it handles the default:

_drop_column_default(conn, table, column)
_alter_column_to_sessionresult(conn, table, column)
_set_column_default_undefined_sessionresult(conn, table, column)

0b1efbb2db84 just omits it. This PR gives it the same order, on both the downgrade and the mirror-image ALTER in upgrade().

Reproduction

On a DB built at head by mgr schema oneshot, downgraded with no skipped revisions:

1. 150 downgrade steps run cleanly.
2. dec0deba5893's downgrade recreates the singular sessionresult and converts sessions.result to it.
3. Crossing 0b1efbb2db84, the has_singular and not has_plural branch fires and aborts.

Confirmed by hand that DROP DEFAULT → ALTER COLUMN TYPE → SET DEFAULT completes, leaving result as sessionresults with default 'UNDEFINED'::sessionresults.

Verification

With this fix on top of #12881, #12883, #12887, #12907, #12910, a single straight alembic downgrade from head to d86417a6bf7b — below the deepest RBAC backfill — now completes:

exit=0   steps run: 175   (no skipped revisions)
current: d86417a6bf7b

That path previously stopped after 28 steps. Each fix moved it further: 28 → 84 → 150 → 175, target reached.

Found while surveying the downgrade chain for #12881.

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